This invention relates to a method of fabricating a high-frequency piezoelectric resonator and in particular a filter.
U.S. Pat. No. 3,694,677 discloses a piezoelectric resonator in which a quartz disk is provided with a thinned central region forming a diaphragm, said thinned region being joined to the non-thinned region by a zone which forms a step. An electrode extends from the exterior of the disk across the stepped region to the center of the thinned region. There is placed on the other face of the disk another electrode which also extends from the periphery of the disk to the center of the thinned region. Said other face can be flat or else it can also have a stepped zone which must pass over the electrode.
It is found in practice that the fabrication of electrodes by sputtering or evaporation of a metal film-layer does not prove satisfactory. The fabrication of these electrodes must in fact meet a number of essential requirements. In the first place, the electrodes must have an electrical resistance which is as low as possible and must also be capable of withstanding the vibrations to which the crystal is subjected. In the second place, the electrodes must not have an excessive mass in order to avoid any risk of producing considerable changes in the frequency of the crystal.
In practice, the electrodes employed are of silver or preferably of aluminum since this is a relatively lightweight material having good electrical conductivity. The thickness of the electrodes is of on the order of approximately 1000 Angstroms, thus making it possible to obtain a resonator having a high Q factor or loaded quality factor.
If the electrodes are fabricated in accordance with the method taught by U.S. Pat. No. 3,694,677, it is found that they have a brittle point at the level of the step. This means that, after a certain period of use and sometimes even immediately after the aging treatment usually performed in order to stabilize the resonator, this latter becomes unserviceable as a result of interruption of electrical continuity.
The production of a reliable piezoelectric resonator therefore makes it necessary to modify the fabrication of the electrodes in order to ensure satisfactory service life of said electrodes.
Furthermore, the present Applicant has found that any accumulation of material at the level of the stepped zone induces stresses in the thinned region of the crystal. These stresses impair the thermal drift of the crystal.